Conventionally, a hybrid vehicle is known that incorporates an engine and an electric motor as driving sources for the vehicle, which are changed over for use in response to a traveling condition of the vehicle. In particular, when the vehicle starts or travels at a low speed, the electronic motor that has a characteristic that it outputs high torque in a low rotational region is used, and then at a stage at which the vehicle speed becomes somewhat higher, the engine that has a torque characteristic favorable in a high rotational region is used. By selectively using the driving sources in accordance with the torque characteristics, the vehicle can be driven to travel efficiently under all conditions.
Incidentally, in a power train of such a hybrid vehicle as described above, a clutch is disposed on a power transmission path from the engine to a driving wheel, and the electric motor is connected to the power transmission path on the driving wheel side with respect to the clutch. The clutch is controlled to a released state, namely, to a disengaging state, when the engine is not used. When the engine is used, the clutch is controlled to a connected state, namely, an engaging state. Further, when the clutch engagement/disengagement state is to transit from the released state to the connected state, control for synchronizing the rotational speed of the input side and the output side of the clutch is carried out.
For example, Patent Document 1 (Japanese Patent Laid-Open No. 2005-130564) discloses a hybrid vehicle that includes a coupling that functions as a clutch. According to this technology, the coupling is connected after the rotational speed of the input shaft side and the rotational speed of the output shaft side of the coupling are synchronized with each other, and after the coupling is engaged, the torque of the electric motor and the engine is controlled. Similarly, also in Patent Document 2 (Japanese Patent Laid-Open No. 2007-320388), when the rotational speeds of a rotational element on the engine side and a rotational element on the motor side both built in a clutch become substantially equal to each other, control of engaging the rotational elements with each other is carried out. Such control prevents occurrence of a significant torque shock due to changeover between the connected and released states of the clutch.
However, in such a conventional hybrid vehicle as described above, since the output motive power of the engine is controlled after the clutch is connected substantially fully, a slow-going feeling or an idly traveling feeling in acceleration sometimes occurs immediately after changeover between the connected and released states is carried out. For example, if the accelerator pedal is operated while the clutch is in a released state, then there is the possibility that the torque transmitted to the driving shaft may be insufficient in comparison with the torque required by the driver. This state wherein the torque is insufficient continues until a state wherein the clutch is connected substantially fully is established.
On the other hand, in order to eliminate such insufficient torque as described above, it seems advisable to vary the output motive power of the electric motor or the engine in accordance with the torque required by the driver. However, the variation of the output motive power of the electric motor or the engine makes it difficult to cause the rotational elements of the clutch to rotate synchronously. Accordingly, a torque shock is liable to arise and the clutch cannot be connected smoothly. Further, the control time to change the state of the clutch between the connected and released states is sometimes elongated.